This invention relates to an apparatus for cutting hollow pipes, particularly spirally formed, hollow metal pipes.
Hollow metal pipes are widely used for ventilation ducts. These pipes are formed from a continuous strip of thin metal. My U.S. Pat. No. 4,567,742, issued Feb. 4, 1986, describes a preferred machine for making triple-ribbed, spiral seam pipe. U.S. Pat. No. 3,132,616 (Hale) describes another type of machine for making corrugated, spiral seamed pipe. Both types of machine start with a flat strip of metal. The strip passes through a series of rollers which bend the edges into predetermined shapes, and form parallel corrugations or reinforcing ribs in the strip. The strip then passes around the inner surface of a forming head (a mandrel) in a spiral manner, so that the strip takes a spiral shape with opposite edges of the strip meshing. The intermeshed edges of the strip are then compressed to form the pipe with a spiral lockseam. The strip is continuously fed into the machine to continuously produce spiral seamed pipe. The spirally formed pipe moves away from the forming head in the axial direction of the pipe and rotates.
At some point the pipe will reach its desired length and must be cut. The assignee of this invention in the past used a rotary saw for cutting the pipe. The saw is mounted on or near the machine, outside the pipe, where it does not interfere with the pipe forming process. When the pipe is ready to be cut, it stops moving. The saw blade is then moved into its cutting position and penetrates the pipe. The metal strip material and pipe forming operation then begin again at a slow speed. This causes the pipe to move forward and rotate. The saw is adapted to move with the pipe for one complete rotation, whereupon the pipe is completely severed. The saw is then returned to its starting position clear of the tube, and the cut pipe section is discharged onto a run-off table. The tube forming process is then repeated to produce another section of pipe.
Hale also discloses a saw blade disposed outside of a spiral seamed, hollow pipe for cutting the pipe into sections.
There are several disadvantages to using high speed saws to cut metal pipes. First, the saw blade, which rotates as fast as 5000 RPM, is dangerous to the machine operator and anyone near the machine. Second, the saw generates a lot of sparks when it cuts the metal, which also creates a hazardous situation. Anyone near the machine must wear safety glasses to protect himself from the sparks. Third, the saw cut leaves burrs on the edge of the pipe. These burrs must be filed off by someone, who must be careful not to cut himself on the burrs. Finally, the cutting operation is very noisy.
My U.S. Pat. No. 4,706,481 discloses a cutting apparatus for hollow metal pipes that overcomes the disadvantages of high speed saw blades. In this apparatus, a first rotatable cutting blade is to be positioned inside a hollow metal pipe. A second rotatable cutting blade is positioned outside of the pipe. The second blade is moved towards the first blade so that the blades overlap to puncture the pipe surface. The blades are adapted to move together in the direction of the pipe, so that they will cut the pipe perpendicularly to the pipe axis as the pipe moves forward and rotates between the overlapping edges of the blades.
The pipe cutting apparatus disclosed in U.S. Pat. No. 4,706,481 uses a slitter-type shearing action, like scissors, to cut the pipe. The pipe is cut without the sparks, noise and danger of a high-speed saw blade. Thus, this apparatus provides a safer environment for manufacturing spiral seamed pipes. Further, this type of cutting apparatus does not leave burrs on the ends of the cut pipe sections. This increases the efficiency of the pipe forming process because manpower does not have to be utilized to de-burr the cut pipes. Still further, this pipe cutting apparatus may be easily adapted to existing spiral pipe producing machines.
Notwithstanding all of its advantages, it has been found that the pipe cutting apparatus disclosed in U.S. Pat. No. 4,706,481 has some limitations. The most notable limitation is that difficulties arise in attempting to cut pipes smaller than two inches in diameter. In this cutting apparatus, an inner rotatable knife and a boom carrying that knife must fit inside the pipe. It has been found that when the boom diameter becomes too small it is difficult to keep the boom rigid during the cutting process. If the boom is not kept rigid, the inner knife may lose its overlapping, slitting relationship with the outer knife. As a result, the pipe may not be completely cut, or the cut may be irregular or have jagged portions.